Handle stopped main event loops in async_to_sync

[davitacols]

Summary

• only reuse the threadlocal main_event_loop when it is still running
• fall back to a new loop if that threadlocal loop has stopped but not closed
• add a regression test covering the stopped-but-open loop case

A stopped loop can still accept call_soon_threadsafe() without ever running the scheduled task, which leaves async_to_sync() blocked waiting for a result.

Fixes #545.

Testing

• python -m pytest -q
• python -m mypy asgiref tests setup.py docs/conf.py